The Britain-based Syrian Observatory for Human Rights said it was not clear whether the militants were preparing to blow up the ancient desert city, a UNESCO Wold Heritage Site, or simply intended to impede any counter offensive by government forces.

Islamic State captured Palmyra from the forces of President Bashar al-Assad in late May, arousing fears that it might destroy its ruins, which include monumental streets lined with columns, temples and burial towers.

The extremist group has previously destroyed ancient monuments in Iraq, saying they were pagan, and is also thought to earn money from selling or authorizing the sale of archaeological artefacts.

But it has also shown off monuments such as the ancient city of Rasafeh, north-east of Palmyra, in propaganda footage about its self-proclaimed caliphate.

Shortly after capturing Palmyra, Islamic State blew up its military prison, notorious for the brutal treatment of political prisoners.

The Observatory, whose reports are compiled from a network of activists around Syria, said that clashes were taking place between Islamic State and government forces on the edge of the desert some 100 kilometres west of Palmyra.

Meanwhile, the Observatory reported further advances by the Kurdish People's Protection Units (YPG), who are pushing south after capturing the border region of Tel Abyad from Islamic State last week.

The YPG victory linked up the Kurdish stronghold of north-eastern Syria with the Kobane enclave and cut off one of Islamic State's main supply routes from the porous Turkish border.

The Syrian conflict, which started when government forces brutally repressed peaceful protests in 2011, has degenerated into a multi-sided civil war between the al-Assad regime, an array of mainly Islamist rebel groups, Islamic State and the Kurds.